Method of operation of a continuous strip heating furnace



Jan. 27, 1970 1 E. sl'MoNclc 3,492,378

METHOD OF OPERTION OF A CONTINUOUS STRIP HEATING FURNACE Filed May 13,1968 United States Patent O 3,492,378 METHOD OF OPERATION OF ACONTINUOUS STRIP HEATING FURNACE Louis E. Simoncic, Walnut Creek,Calif., assignor to Bethlehem Steel Corporation, a corporation ofDelaware Filed May 13, 1968, Ser. No. 728,549 Int. Cl. F27b 9/28, 9/12;C21d 1/26 U.S. Cl. 263-52 2 Claims ABSTRACT OF THE DISCLOSURE BACKGROUNDOF 'II-IE INVENTION This invention relates to an improved method ofoperating a metal heating furnace and more particularly to a method ofoperating a vertical furnace for the continuous heating of steel stripby means of open-flame burners.

Cold reduction of steel strip produces a product which is hard, haslittle ductility, and is unsuitable for further metalworking. Coldreduced steel can be restored to a soft ductile state by the process ofheat treating. The heat treating operation comprises the steps ofheating the steel to a high temperature and then cooling the stripaccording to a predetermined time temperature relationship, which mustbe closely controlled to permit maximum production of high qualitystrip. Early heat treating furnaces were of the batch type in whichsteel, in the form of packs of sheets or in coils, was first heatedbeneath a box-type hood and then cooled slowly in place. Today, a largeamount of steel strip is heat treated in continuous lines, which includea heating furnace or zone that is followed by one or more cooling zones.Strip traveling at a high rate of speed continuously passes through thefurnace where the steel is heated to the desired temperature and thenproceeds through a cooling zone or zones where the steel is cooled in acontrolled maner. In most instances, particularly where the product isto be coated with molten zinc or the like, great care is taken to insurethat the steel strip leaving a heat treating line has a cleanbrightsurface, i.e. unoxidized.

By controlling the atmosphere in the several zones of a continuous heattreating line a clean scale-free product can be produced. In the coolingzones of such lines the atmosphere is controlled by introducing apassive gas, such as a 95%-5% mixture of nitrogen and hydrogen, intosuch zones. In the furnace or heating zone of such lines the manner ofcontrolling the atmosphere depends upon the method by which such zonesare heated. When the furnace zone is indirectly fired, as by means ofradiant tube heaters, the atmosphere is controlled by introducing apassive gas into the heating zone. When the furnace zone is directlyred, as by means of open-ame burners, the atmosphere comprises theproducts of combustion from the burners. The atmosphere in direct firedfurnaces is controlled to maintain it deficient in oxygen, i.e.essentially non-oxidizing for heat treating purposes, by firing all ofthe burners with an air-gas mixture deficient in oxygen.

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Many direct fired heating furnaces fo continuous heat treating lines arevertical structures which are divided into several zones. Each zone isfired by a plurality of burners arranged on both sides of the stripmaterial which passes downwardly through the furnace. Firing all burnersin such furnaces with a fuel-air mixture deficient in oxygen has beenrecognized as an inefficient method of heating. However, this manner ofoperation has been considered necessary to insure that an essentiallynon-oxidizing atmosphere was ymaintained throughout the heating zone. Inaddition to being an ineicient method of heating, this method ofoperation causes other problems. The oxygen deficient products ofcombustion pass upwardly through the furnace and into the stack wherethey cornbine with oxygen from air drawn into the stack and burn,thereby raising the stack temperature with resultant damage to the stackbrickwork.

SUMMARY OF THE INVENTION It is an object of this invention to moreefficiently operate a direct tired vertical continuous heating furnace`than the method heretofore in use.

Another object of this invention is to operate a direct tired verticalcontinuous heating furnace in a manner to reduce the temperature in itsstack and prolong the life of the stack refractory.

The present invention provides a novel method of operating a direct redvertical furnace for the continuousy DESCRIPTION OF THE DRAWINGS FIGUREl is a transverse sectional view of a heating furnace or chamber of acontinuous heat treating line.

-FIGURE 2 is an enlarged fragmentary view illustrating the constructionof the burners in one of the heating zones of the furnace of FIGURE 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings thereis shown a vertical furnace or heating chamber 10 of a continuous heattreating line 11. Furnace 10 is forme-d of two panels 12 `and 13 whichcan be separated to permit access to the interior portions for repairsand for other purposes, such as threading strip material through thefurnace. Furnace 10 is divided into several heating zones, zones 14, 15and 16 in the lower portion of the furnace and zone 17 in the -upperportion of the furnace. Chamber or space 18 between the opposing facesof the zones is made as narrow as practicable for most efficientheating. Each zone is heated by a plurality of opendiame gas burners 19,of the radiant refractory type, arranged in horizontal rows. Each burner19 comprises a block 20 of refractory material having a concavedepression 21 and a central passage 22. A burner tube 23 extends througheach passage 22 and terminates in a nozzle 24 the end of which projectsinto the bottom of its respective block depression 21. The burners 19 ofeach row receive a mixture of combustible gas and air from `a horizontalmanifold 2S which connects with a vertical supply pipe 26 that connectswith a supply source, not shown. Each zone has a Separate supply pipe inwhich there is a mixing valve that controls the ratio of air to fuel ofthe combustible mixture supplied to the zone burners.

Continuous metal strip 27 to be eat treated passes over guide roll 28and into furnace 10 through opening 29 in the furnace top. The strippasses vertically downward through chamber 18 between opposite banks ofburners 19 in each of zones 17, 16, 15 and 14 and is progressivelyheated to the desired temperature. The strip leaves the furnace at thebottom through opening 30, passes under guide roll 31 and continuesthrough the remainder of the heat treating line, not shown. Hot productsof combustion from burners 19 rise through furnace chamber 18, and arecarried away by ilues 32 to stack 33. Adjacent top opening 29 are pipes34, one on each side of strip 27, through which compressed air is blowndownwardly into the opening. The compressed air acts as a seal andprevents the escape of hot gases through opening 29.

To obtain maximum heat output from a burner there must be astoichiometric mixture of oxygen and fue-l for the particular fuel used.This ratio will vary according to the particular fuel used. For example,oxygen and methane react as follows:

The reaction is exothermic. A stoichiometric mixture occurs and maximumheat is produced when two molis of oxygen react with one mol of methane,an oxygen-methane ra-tio of 2 to l. When natural gas, which is primarilymethane, is used as a fuel, an air to fuel ratio of approximately 10.5to 1 provides maximum heat.

In the operation of furnace according to my invention burners 19 in thelower portion of the furnace, i.e. zones 14, and 16, are tired with acombustible air-fuel mixture which has a deficiency of oxygen and whichwill yield near maximum heat, while producing products of combustionthat are deficient in oxygen, i,e, essentially nonoxidizing. As is wellknown to those skilled in the art, the non-oxidizing nature of suchproducts of combustion is a function of the differential between t-hellame temperature of the burners and the temperature of the strip, aswell as the ratio of both CO to CO2 and H2 to H2O. In the upper portionof the furnace, i,e. in zone 17, burners 19 are red with a combustibleair-fuel mixture which has an excess of oxygen and which will yield nearmaximum heat value, while producing an atmosphere having an excess ofoxygen. This oxygen in upper zone 17 provides the additional oxygennecessary to complete combustion in this zone of the oxygen deficientproducts of combustion which pass upwardly from lower zones 14, 15 and16. The energy thus released is available to usefully preheat the stripin the portion of the furnace above zone 17.

In practice the burners of furnace 10 are red with a combustiblemix-ture of air and natural gas having a heat content of 1035 to 1062B.t.u./cu. ft. In the lower portion of furnace 10, burners 19 are firedwith an air-gas mixture deficient in oxygen. In zone 14 the burners arefired with an air-gas mixture having a ratio of about 71/2 to 1; in zone15, the burners are fired with an air-gas mixture having a ratio ofabout 8 to 1; and in zone 16 the burners are red with an air-gas mixturehaving an air-gas mixture of about 9 to 1. These air to gas ratios arelower ,than the approxi-mate 10.5 to 1 stoichiometric ratio for amixture of air and natural gas. In the upper portion of the furnace, inzone 17, burners 19 are fired with an airgas mixture of about 11 to 1,i.e. having a slight excess of oxygen and greater than thestoichiometric ratio for a mixture of air and natural gas. The resultingproducts of combust-ion having an excess of oxygen formed in zone 17,along with air which infiltrates into the furnace, provide oxygen tocomplete combustion of the oxygen deficient products of combustion thatrise from the lower zones.

By operating the furnace by the method of this invention the atmospherewithin the furnace is maintained essentially non-oxidizing andsubstantially all of the gases are burned within the furnace, prior totheir passing into ilues 32 and stack 33. The fuel balance of thefurnace is more efficient than the efficiency obtained by the priormethod of operating such furnaces, and the strip may be passed throughthe furnace at a faster speed than was previously the case.

What is claimed is:

1. A method of operating a vertical furnace for 'the continuous heatingof uncoated metal strip, said furnace comprising an upper portion and alower portion, each of said portions having a plurality of burners, saidmethod comprising the steps of l(A) passing the strip downwardly throughsaid furnace,

(B) heating the strip by tiring substantially all of said burners ineach said upper and lower portions with a combustible mixture of air andfuel,

(l) the combustible mixture supplied to the burners in the upper portionof said furnace having an air to fuel ratio greater than thestoichiometric ratio,

(2) the combustible mixture supplied to the burners in the lower portionof said furnace having an air to fuel ratio less than the stoichiometricratio.

2. A method of operating a verticai furnace for the continuous heatingof uncoated metal strip, said furnace comprising an upper portion havinga first heating zone, and a lower portion comprising a second heatingzone and a third heating zone located below said second zone, each ofsaid zones having a plurality of burners, said method comprising thesteps of (A) passing the strip downwardly through said furnace,

(B) heating the strip by ring substantially all of the burners in eachzone With a combustible mixture of air and fuel,

(l) the combustible mixture of air and fuel supplied to the burners inthe upper portion rst heating zone having an air to fuel ratio greaterthan the stoichiometric ratio,

(Z) the combustible mixture of air and fuel supplied to the burners inthe lower portion second heating zone having an air to fuel ratio lessthan the stoichiometric ratio,

(3) the combustible mixture of air and fuel supplied to the burners inthe lower portion third heating zone having an air to fuel ratio lessthan the stoichiometric ratio and less than the air to fuel ratio of thecombustible mixture supplied to the burners in the lower portion secondheating zone.

References Cited UNITED STATES PATENTS 2,625,387 1/1953 Hess 263-52 JOHNI. CAMBY, Primary Examiner U.S. Cl. X.R. 148-16

